The homeostatic cytokine IL-7 is currently under clinical investigation as a potential immune-reconstitution agent in various forms of immunodeficiency, including HIV infection and cancer. We previously performed a study of IL-7 treatment during the acute phase of SIV infection in a well-established nonhuman primate model of AIDS. Strikingly, we found that animals treated with IL-7 were protected from the dramatic loss of circulating naive and memory CD4(+) T cells that typically occurs in untreated animals upon SIV infection. This effect was associated with the development of earlier and stronger SIV Tat-specific T-cell responses. Thus, we demonstrated that IL-7 administration is effective in protecting the CD4(+) T-cell pool during the acute phase of SIV infection in macaques, providing a rationale for the clinical evaluation of this cytokine in patients with acute HIV-1 infection. In contrast to the beneficial effects that we documented in acute SIV infection, subsequent studies from our laboratory demonstrated an opposite effect of IL-7, which may paradoxically foster the progression of HIV-1 disease toward full-blown AIDS. We observed that IL-7, in the absence of any concomitant stimulation, potently and selectively induces the expression of the principal gut-homing integrin, alpha4beta7, in both CD4+ and CD8+ T cells. We found that this effect: i) is specific for T cells; ii) is rapidly induced upon IL-7 treatment; iii) requires supra-homeostatic concentrations of IL-7 (those typically reached under conditions of lymphopenia); iv) is uncoupled from the expression of classic markers of cellular activation; and v) is associated with the functional activation of the integrin, as indicated by an increased binding activity for its natural ligand, MAdCAM. Investigation of the molecular mechanisms of a4b7 induction by IL-7 revealed the involvement of both major signaling pathways linked to stimulation of the IL-7 receptor, i.e., the JAK/STAT and PI3K/Akt pathways. Induction of a4b7 by IL-7 was also confirmed in vivo, both in HIV-infected subjects and in SIV-infected macaques treated with IL-7. Of note, we found that induction of a4b7 by IL-7 occurs predominantly in phenotypically naive T cells, which concomitantly acquired a memory-like phenotype, as shown by upregulation of CD95 expression and secretion of TNF-a; upon stimulation with phorbol esters and ionomycin, despite an unaltered expression of CD45RA and CD45RO. This memory-like masquerade of T cells is similar to that previously documented in vivo in naive T cells of mice recovering from lymphopenia. Naive T cells were also induced to proliferate by IL-7, in the absence of any concomitant stimulation, albeit with delayed kinetics compared to a4b7 induction. These results are compatible with a new model of host response to lymphopenia whereby supra-homeostatic levels of IL-7 activate an unusual program of phenotypic modulation in naive T cells, characterized by the acquisition of a gut-homing and memory-like phenotype prior to the induction of cell cycling and proliferation. The role of intestinal T-cell homing in the reconstitution of the depleted T-cell pool in lymphopenic hosts remains to be defined. To formally demonstrate the physiological relevance of a4b7 induction by IL-7, we performed an in vivo study in which humanized NSG mice were injected with autologous T cells treated or not with IL-7. The results of these experiments clearly documented a preferential intestinal homing of IL-7-treated naive T cells, while no preferential homing to other tissues was detected. The physiological relevance of these phenomena in the processes of immunologic reconstitution is currently under investigation. Because endogenous levels of IL-7 naturally increase to supra-homeostatic concentrations in response to lymphopenia during the progression of HIV-1 disease, we hypothesized that the ability of IL-7 to redirect naive T cells to the intestinal compartment could occur in vivo in individuals with progressive HIV-1 disease. To investigate these phenomena and their relevance to AIDS in an in vivo model, we designed a new study in which 6 macaques chronically infected with either SIVmac251 or SIVsmE543 received a single injection of IL-7 (50 ug/kg, s.c.) and were sacrificed seven days later in order to specifically investigate the effects of IL-7 on T-cell homing and SIV replication in peripheral lymphoid tissues, particularly the GALT; three chronically SIV-infected animals received placebo and served as untreated controls. Detailed phenotypic analysis of circulating T cells documented a rapid upregulation of a4b7 in both CD4+ and CD8+ T cells. Comparison of pre-treatment and post-treatment intestinal tissues demonstrated that IL-7 administration resulted in increased numbers of infiltrating T cells within the GALT, associated with increased levels of SIV replication, predominantly in the Peyers patches. Enhanced SIV replication was also detected in lymph nodes. The increased levels of SIV replication in peripheral lymphoid tissues were mirrored by consistent increases in SIV plasma viremia. These in vivo data provide an initial validation of our hypothesis that the surge of endogenous IL-7 that occurs during the late stages of HIV infection may foster the terminal depletion of the CD4+ T-cell pool through the induction and activation of a4b7 leading to increased intestinal homing and HIV susceptibility. We also investigated the molecular mechanisms underlying the unique physiological effects of IL-7 on different T-cell populations, with particular emphasis on the ability of this cytokine to induce the expression of integrin a4b7 and the unconventional cold activation state in naive T cells. Thus, we used microarray analysis of mRNA transcripts from highly purified subpopulations of CD4+ naive, memory a4b7high and memory a4b7low T cells, and tested them before and after stimulation with suprahomeostatic levels of IL-7. The results provided new mechanistic insights into the activity of IL-7, showing a rapid upregulation of specific genes that regulate cytokine networks and cytokine receptor expression. If these microarray results are validated by real-time PCR and siRNA knock-down, some of these IL-7-regulated genes may become suitable targets for new strategies of immunomodulation therapy.